This invention relates to silicon carbide and in particular, to a method of manufacturing a film of silicon carbide, a device including the silicon carbide, an ingot of silicon carbide, and the like. In this event, it is to be noted that the silicon carbide is used for a substrate material of a semiconductor device, a sensor, a dummy wafer in a semiconductor manufacturing process, an X-ray mask, a solar cell, and so on.
It is known in the art that silicon carbide itself is a semiconductor which has a forbidden band as wide as 2.2 eV or more and which is formed by thermally, chemically, and mechanically stable crystals. In addition, consideration has been made about applications of the silicon carbide to a semiconductor substance which is used on conditions of a high frequency and a high electric power because the silicon carbide has a high thermal conductivity. As a method of manufacturing the silicon carbide, have been known in the art the Acheson method and the sublimation and recrystallization method (will be also called an improved Lely method). Specifically, the Acheson method is for reacting silicon on heated coke to deposit the silicon carbide on the surface of the coke while the sublimation and recrystallization method is for heating the silicon carbide obtained by the Acheson method to sublimate and thereafter recrystallize it. In addition, is also known a liquid deposition method which melts silicon within a carbon crucible to pulling the silicon carbide with reacting floating carbon in the crucible with the silicon.
Moreover, any other methods have also been proposed so as to obtain a silicon carbide film which has a high purity and reduced crystal defects. Specifically, as such methods, have been known a chemical vapor deposition (CVD) method and an atomic layer epitaxy (ALE) method. In the CVD method, the silicon carbide is deposited on a surface of a substrate by thermally reacting a carbon source gas with another silicon source gas in a normal or a reduce pressure atmosphere. On the other hand, silicon source molecules and carbon source molecules are alternately adsorbed on a substrate surface and epitaxial growth of the silicon carbide proceeds with crystallinity of the substrate kept unchanged in the silicon carbide.
Herein, it is to be noted that, when the silicon carbide is used as a material of a semiconductor device, controlling an impurity is extremely important. For example, let the silicon carbide be used as a substrate for a power semiconductor device of a discrete type, such as a Schottky-barrier diode. In this event, the device has a series resistance or an on-resistance when the device is put in an on-state and the on-resistance is preferably small because of a reduction of a power loss within the device. In order to decrease the on-resistance, the substrate must be doped with an impurity of an amount as large as 1021/cm3 at maximum.
On the other hand, consideration should be made about a breakdown voltage of a semiconductor device. Such a breakdown voltage of the semiconductor is generally proportional to −0.5 power (namely, minus square root) of the impurity concentration. Taking this into account, the impurity concentration should be reduced to 1×1014/cm3 at a portion of the device at which an electric field is concentrated.
In the meanwhile, a thermal diffusion method is used to dope the impurity into the substrate on manufacturing the semiconductor device which uses silicon as a base material. The thermal diffusion method is for adding the impurity into the substrate by coating an impurity on a substrate surface or by exposing the substrate in an impurity atmosphere and by thereafter heating the substrate. However, such a thermal diffusion method can not be applied to a silicon carbide substrate. This is because a diffusion coefficient within the silicon carbide is extremely slow as compared with that within the silicon. This make it very difficult to diffuse an impurity to a depth (deeper than 1 μm with a concentration range between 1×1014 and 1×1021/cm3) which is available for manufacturing the semiconductor device.
Under the circumstances, an ion injection method is usually used to add an impurity to silicon carbide and is useful to widely control an impurity concentration. However, restriction is inevitably imposed in the ion injection method on a distribution of impurity along a depth direction due to a range of injected ions. In other words, the distribution of impurity depends on the range of the injected ions. Taking this into consideration, Japanese Unexamined Patent Publication No. Hei.11-503571, namely, 503571/1999 discloses a method of introducing a dopant into a semiconductor layer of silicon carbide. More specifically, the method should have a step of ion injecting a dopant into a semiconductor layer at a low temperature and a step of annealing the semiconductor layer at a high temperature. In this event, the ion injecting step is performed at the low temperature so that an amorphous layer is formed near to a surface of the semiconductor while the annealing step is performed at the high temperature so that the dopant is diffused into an un-injected layer laid under the amorphous layer. Even when this method is used, it is difficult to diffuse the impurity with a high concentration over a whole of the substrate.
In addition, injected ions are insufficient with electrical activation and the ion injection brings about the crystal defects within the silicon carbide. Under the circumstances, proposal has been made in Japanese Unexamined Patent Publication No. Hei 12-068225, namely, 068225/2000 about a method of additionally ion injecting carbon atoms (C) to improve electrical activation of acceptors injected into the silicon carbide. This method is also effective to suppress diffusion resulting from heat treatment. Furthermore, Japanese Unexamined Patent Publication No. Hei 11-121393, namely, 121393/1999 discloses a method of forming a mask of SiO2 on a silicon surface of a silicon carbide substrate and thereafter carrying out ion injection of nitrogen as impurity element. After injection of the impurity, this method should further carry out ion injection (channeling injection) from a direction perpendicular to the silicon surface and another ion injection (random injection) from another direction oblique from the perpendicular direction by 7 degrees. As pointed in Japanese Unexamined Patent Publication No. Hei 11-121393, when phosphorus atoms are ion injected into the semiconductor of silicon carbide, the temperature on the ion injection should be kept at a high temperature, such as 1200° C. or more.
Herein, let an impurity be added all over a substrate. In this case, use is made of a method which forms silicon carbide simultaneously with doping an impurity and which may be called in-situ doping. In such in-situ doping, restrictions are inevitably imposed on an impurity source and a concentration to be added. For example, disclosure is made in Japanese Unexamined Patent Publication No. Hei 09-063968 about a method which causes a boron inclusion gas to flow simultaneously with feeding a mix gas of carbon and silicon and which serves to grow a semiconductor layer of p-silicon carbide in a vapor phase. In this event, when a supply quantity of carbon and a supply quantity of silicon, both of which contribute to crystal growth, stand for QC and QSi, respectively, the following relationship should hold:1<QC/QSi<5.
As regards the semiconductor layer of the p-silicon carbide deposited in the above-mentioned manner, the following relationship between atomic density dC of the carbon and atomic density dSi of the silicon should be satisfied:1<dC<dSi<32/31.
As mentioned in Japanese Unexamined Patent Publication No. Hei 10-507734, namely, 507734/1998, trialkylboron should be used as an organic boron compound in a CVD process or a sublimation process. Specifically, let use be made of the organic boron compound which has, in a molecule, at least one boron atom chemically bonded to at least one carbon atom, when doping is carried out in a single crystal of silicon carbide by each of the CVD and the sublimation process. The above-mentioned Publication points out that trialkylboron effectively acts as such an organic boron compound.
In order to vary a concentration of nitrogen as an impurity over a wide range by using in-situ doping technique, Applied Physics letters 65(13), 26 (1994) reports about varying a concentration of carbon which competes with nitrogen in an occupancy ratio of crystal lattices in silicon carbide. In this case, since the concentration of nitrogen arranged in positions of the crystal lattice in place is sensible against the concentration of carbon, a composition ratio of a silicon source and a carbon source should be strictly controlled on growing the silicon carbide. This makes mass-production of the silicon carbide difficult.
Alternatively, let an impurity be doped with silicon carbide by using the sublimation and recrystallization method. In this event, silicon carbide powder and an impurity source (such as Al, B) which act as raw materials should be mixed at a predetermined ratio and sublimated to be recrystallized on a seed crystal. Herein, it is noted that a vapor pressure of the impurity source is very higher than that of the silicon carbide at a sublimation temperature. In consequence, an impurity concentration in the silicon carbide inevitably becomes high at a beginning of silicon carbide growth and becomes low at an end of the growth because the impurity source is wasted and extinct.
Such a variation of the impurity concentration gives rise to a variation of resisitivity among silicon carbide substrates when the silicon carbide formed by the sublimation and recrystallization method is sliced to obtain the silicon carbide substrates. This makes it difficult to realize a stable characteristic of a device. In addition, the silicon carbide grown by the sublimation and recrystallization method does not always have a flat surface and a sharp pn junction or a flat pn junction can not be attained by the use of such silicon carbide.
In the conventional in-situ doping which dopes an impurity during growing the silicon carbide by a vapor growth method, capturing the impurity proceeds simultaneously with growth of silicon carbide. Taking this into consideration, let a pn junction be formed by the use of the above-mentioned in-situ doping. In this case, impurity materials should be switched from one to another during the doping. On switching the impurity materials, a previous impurity gas is inescapably left in a reaction system at the beginning of doping another impurity. In consequence, it is difficult to obtain a sharp pn junction which has a clear junction boundary between p and n regions. In addition, donor impurities and acceptor impurities coexist in a portion adjacent to the junction boundary and such coexistence brings about a high compensation degree and which makes it difficult to enhance mobility in the pn junction.
Moreover, gas flows and the like give rise to an uneven distribution of impurity concentrations in a plane and a uniform impurity concentration can not be obtained over a wide range. Hence, the impurity concentrations can not be strictly controlled and the silicon carbide which has desired impurity concentration distributions can not be attained with a high yield.